1. Field of the Invention
The invention relates generally to systems, devices and methods for sound reproduction. More specifically, the invention relates to a parametric sound reproduction system wherein economies are realized by dynamically adjusting the ultrasonic carrier level in a parametric array in response to changing input levels of the source audio signal being reproduced in the array.
2. Related Art
It has been recognized that there are advantages in modulating the output power level, or “envelope” (amplitude modulated or single sideband modulated) of an ultrasonic carrier wave in a parametric loudspeaker system application. This has been known since at least as early as 1991, when the work of Kamakura, Aoki, and Kumamoto was published, as noted below. Modulation of the carrier can provide a more efficient system than using a carrier of fixed amplitude, as such a fixed carrier must be of sufficient amplitude level to accommodate peak levels in the audio source material signal without distortion. In contrast to a fixed carrier, using a modulated carrier the envelope can expand and contract with the source signal level; and it is possible to produce a carrier amplitude of essentially zero when the source signal level is essentially zero, for example. Average radiated power is markedly reduced because of the greater efficiencies inherent in only providing so much carrier amplitude as is needed to accommodate the source signal level. Accordingly, less amplifier power is required, and less emitter heating is caused, both enabling lower costs in the system. Attempts have been made to accomplish this variation of radiated power of the carrier in a variety ways.
Examples of such prior work, and further background information regarding parametric array systems and carrier modulation can be found in the following references: published European Patent Application No. EP 0973152 A2 filed Jul. 15, 1999 by Massachusetts Institute of Technology, naming Frank J. Pompei as inventor; published European Patent Application No. EP 0003931 A1 filed May 5, 2000 by Sennheiser Electric GMBH & CO.KG, naming Wolfgang Niehoff et al. as inventors; and, the article referenced above, “Suitable Modulation of the Carrier Ultrasound for Parametric Loudspeaker” by T. Kamakura, K. Aoki, and Y. Kumamoto, ACUSTICA Vol. 73 (1991), each of these references are incorporated in this disclosure by reference for the relevant teachings consistent with this disclosure.